A heat exchanger with an outer pressure jacket, U-shaped pipes, and an intermediate guide jacket is known from DE-PS 20 33 128. In this known heat exchanger the guide jacket is open at both ends and is provided in the middle with a feed connection for the first medium flowing through the jacket. The first medium, which is the medium to be cooled, is distributed within the guide jacket to both sides to flow partly parallel to the second fluid medium in the heat exchange pipes and partly in counterflow thereto. Thereafter, the first medium in the jacket enters into the annular space between the guide jacket and the outer pressure jacket from which it is subsequently drained. In this manner, the pressure jacket does not come into contact with medium of high entry temperature.
In some applications, it is desired to control the temperature of the medium to be cooled and flowing inside the jacket. To achieve this goal, a bypass arrangement is known (DE-PS 28 46 455) which consists of a central pipe that is positioned parallel to the longitudinal axis of the heat exchanger. This central pipe directly connects a hot gas entry chamber with a cooled hot gas exit chamber and at one end houses a closing or regulating member. This bypass arrangement is only usable in straight pipe heat exchangers wherein the heat exchange pipes are held in two pipe supporting floors which respectively delimit the gas entry chamber and the gas exit chamber.